This invention relates to a method for fabricating a solid-state imaging device in which scanning circuits (scanners) and a photoconductive film are integrated on a semiconductor substrate.
As potent systems for constructing solid-state imaging devices, there have been considered the two types of the CCD type (Charge Coupled Devices) and the MOS type (a device in which the source junction of a switching insulated-gate field effect transistor is utilized as a photodiode). All these devices have the advantage that they can be fabricated with the MOS process technology of high integration density. Since, however, a photosensitive portion lies under an electrode (in the case of the CCD type) or on the same plane as that of switching transistors as well as a signal output line (in the case of the MOS type), the devices have the disadvantage that the incidence of light is hindered in many regions by the electrode or the switching transistors, in other words, that the optical dead region is large. Further, since the photosensitive area and the scanner area are even as described above, the occupation area of picture elements becomes large, that is, the integration density of picture elements cannot be made high, resulting in the problem that the resolution cannot be made high.
As a structure for solving these drawbacks (sensitivity, resolution), the inventors have provided a solid-state imaging device of the double-layer structure in which a photoconductive film is disposed on a scanner IC (Japanese Laid-open Patent Application No. 51-10715, or `Technical Digest of Electron Devices Meeting 1979 Dec. 3-4-5`, pp. 134-136, T. Tsukada et al.). By taking as an example a case where the double-layer solid-state imaging device is constructed of MOS type elements, the outline of the device structure is shown in FIG. 1A. The arrayal pattern of picture element electrodes 9 (9-1, 9-2, 9-3, 9'-1, 9'-2, 9'-3) is shown in FIG. 1B. Numeral 1 designates a semiconductor substrate of a first conductivity type, and numeral 2 an insulated-gate field effect transistor (hereinbelow, termed "MOST") which constructs a switching element adapted to be turned "on" and "off" by an output of a scanning shift register (not shown) and which consists of a drain 3, a source 4 and a gate 5. Numeral 6 designates a photoconductive film serving as a photosensitive material, numeral 7 a transparent electrode for applying a voltage which drives the photoconductive film, and numeral 8 an insulating film. Shown at numeral 11 is a signal readout electrode. As apparent from FIG. 1A, the double-layer structure is formed of an integrated circuit (IC) in which the semiconductor substrate 1 and the scanning shift register as well as the switching elements 2 are integrated, that is, a scanner IC, and a photosensor which consists of the photoconductive film 6 and the transparent electrode 7. With the solid-state imaging device of FIG. 1, accordingly, the utilization factor of area is high, and the size L per picture element is small, that is, the resolution is high. In addition, since the photosensitive portion lies at the upper part on the side of incident light 10, the optical dead region is not existent and the sensitivity is high. Further, a desired spectral response can be attained by selecting the photoconductive film properly. In such manner, performances which are much more excellent than those of the conventional solid-state imaging devices can be expected.
FIG. 1C shows an example of the construction of the double-layer solid-state imaging device. In the figure, numeral 101 indicates a horizontal scanning shift register, numeral 102 a vertical scanning shift register, numeral 103 a horizontal switching element (MOST) for selecting the horizontal position of a picture element, numeral 104 a vertical switching element (MOST) for selecting the vertical position of the picture element, numeral 105 a photosensor, numeral 106 a driving voltage terminal, numeral 107 a vertical signal output line, and numeral 108 a horizontal signal output line. The MOST 104 corresponds to the switching element 2 in FIG. 1A, and has the picture element electrode 9. The photosensor 105 consists of the photoconductive film 6 and the transparent electrode 7 in FIG. 1A. When the MOSTs are of the n-channel type and the photoconductive film is an Se-As-Te chalcogenide film, a driving voltage to be applied to the terminal 106 is approximately 50 V and a target voltage V.sub.T is approximately 1 V.
Hereupon, the double-layer device has the problem in fabrication that an etchant for etching the materials for use as the photoconductive film has not been found yet, or that even if it is found in the future, the materials cannot be dipped in the etchant because of poor immunities to chemicals (in other words, intense chemical activities). Inevitably, the photoconductive film is disposed on the whole surface of the scanner IC. As a result, the photoconductive film and the transparent electrode are disposed also on the scanning shift register, and the driving high voltage applied to the transparent electrode is feared to bring the scanning shift register into a malfunction.
Image pickup tubes employ a large number of photoconductive materials including Se-As-Te, amorphous Si, PbO, CdTe, CdS, etc. The image pickup tubes, however, require only the step of evaporating the materials on glass plates and do not require the step of forming a pattern (removing the materials in unnecessary regions). Accordingly, the problem concerning the immunity of the photoconductive film against chemicals is a difficulty peculiar to the case of fabricating the solid-state imaging device (double-layer device).